"a solemn face" is a correct and usable phrase in written English.
It is typically used to describe someone's expression or demeanor as serious, grave, or earnest. Example: The room fell silent as the judge entered, his face a solemn mask as he prepared to announce the verdict.
Jaspreet Singh, a young farmer with a solemn face, explains that they are digging an extra 10 feet into the ground every 4 years to reach the water.
Thaci knows what Kouchner expects of him, so he runs through his lines with a solemn face, condemning, regretting, affirming and so on.
One in the show has big hair, a solemn face, wings like cupboard doors, and a pocketbook clutched to her side.
Now 55 and retired from Alabama's Department of Industrial Relations, Fields has a solemn face with chestnut-hued, vaguely Cherokee features, a black mustache, powder-gray hair and the relaxed, affable bearing that doesn't ever seem ruffled in those moments when someone informs him that he is the first black ever to enter their home — which happens to him frequently in Cullman.
Before the game, Hughes spoke to reporters while wearing an ice pack on his shoulder and a solemn expression on his face, the look of a man sick of being stuck in this pitching purgatory, shut down indefinitely but in good health.
Sitting among them, in a dark suit with a solemn expression on his face, was a seriously jet-lagged Mr. Clinton.
Standing at a cliff with a solemn look on his face, he was about to receive his lightsaber from Rey, the young heroine, when the story ended and the credits rolled.
Eighth-Grade Reading (1998) 1. Acceptable (57 percent): "I imagine a girl wearing too much makeup and a boy with a solemn expression on his face as he tries to pay with an orange".
Outside the Dunedin branch of Gun City on Monday, Karen Nielsen, 47, stood holding a white poster board with black capital letters reading, "This store sells weapons of mass destruction". She had a solemn expression on her face as she described how the "horrific, horrific events" in Christchurch had inspired her to protest that morning after she drove her children to school.
He considered the 76 people in this grove, the 49,000 people in his home town of Coeur d'Alene, the millions of people in the United States driving billions of miles a year — and approached his teacher, Jamie Esler, with a solemn look on his face.
Cosby remained seated in the courtroom, surrounded by attorneys and spokespeople, a solemn look on his face.
